1. Field of the Invention
The present invention relates to wireless communications technology. More specifically, the present invention provides a system that reduces interference between a macrocell and a femtocell, and between neighboring femtocells.
2. Discussion of the Related Art
Recently, a new class of base stations which are designed for indoor and personal uses is discussed in (a) an article “UMA and Femtocells: Making FMC Happen”, by Partho Choudhury and Deepak Dahuja published as a white paper, December 2007; (b) “Femto Cells: Personal Base Stations,” published as a white paper from Airvana Inc., 2007, available on-line at http://www.airvana.com/files/Femto_Overview_Whitepaper_FINAL—12-July-07.pdf; and (c) the article “The Case for Home Base Stations,” published as a white paper from PicoChip Designs Ltd., April 2007, available on-line at: http://www.picochip.com/downloads/27c85c984cd0d348edcffe7413f6ff79/femtocell_wp.pdf. As described in these publications, the cells serviced by these personal base stations are referred to as “femtocells;” these femtocells provide indoor connectivity through existing broadband Internet connections. Femtocells are also considered as an option for fixed-mobile convergence (FMC), which enables subscribers to switch an active data call session between a fixed wireless network (e.g., a wireless local area network (WLAN)) and a mobile network (e.g., a cellular network). The benefits of a femtocell include (a) improved indoor coverage, (b) reduced capital and operational expenditure, (c) reduced bandwidth load, (d) reduced power requirements, (e) additional high-end revenue streams, (f) improved customer royalty, (g) increase in the average revenue per user, (h) compatibility with existing handsets, and no requirement of dual-mode terminals, (i) deployment in operator-owned spectrum, and (j) enhanced emergency services (since the femtocell possesses knowledge of its location).
As far as physical layer transmission is concerned, prior art femtocells are most often designed for code division multiple access (CDMA) systems and 3rd Generation (3G) technologies. Such systems are disclosed, for example, in the articles (a) “Uplink Capacity and Interference Avoidance for Two-Tier Cellular Networks” (“Chandrasekhar”), by Vikram Chandrasekhar and Jeffrey G. Andrews, published in Proc. IEEE Global Telecommunications Conference (GLOBECOM), Washington, D.C., pp. 3322-3326, November 2007; (b) “Effects of User-Deployed, Co-Channel Femtocells on the Call Drop Probability in a Residential Scenario” (“Ho”), by Lester T. W. Ho and Holger Claussen, published in Proc. of IEEE Int. Symp. on Personal, Indoor and Mobile Radio Communications (PIMRC), Athens, Greece, pp. 1-5, September 2007; and (c) “Performance of Macro- and Co-Channel Femtocells in a Hierarchical Cell Structure” (“Claussen”), by Holger Claussen, published in Proc. of IEEE Int. Symp. on Personal, Indoor and Mobile Radio Communications (PIMRC), Athens, Greece, pp. 1-5, September 2007.
Chandrasekhar derives and analyzes the uplink (UL) capacity of a femtocell network coexisting with a macrocell network (i.e., a shared-spectrum network). In a split spectrum network, the femtocell users and the macrocell users are assigned orthogonal sub-channels. While orthogonal subchannels avoid interference between the macrocell and the different femtocells, such a scheme decreases the number of users that can be supported. On the other hand, for a shared spectrum network, a femtocell may utilize some sub-channels that are already used by the macrocell, whenever there is little or diminished interference between the two networks. To reduce outage probability, Chandrasekhar proposes using interference avoidance methods. Specifically, the macrocell users and the femtocell users use time-hopping to decrease interference. Furthermore, both the macrocell and the femtocell use sectored antenna reception to increase capacity. Analytical and simulation results show that, through interference avoidance (e.g., time-hopped CDMA and sectorized antennas), up to seven times higher femtocell base station (fBS) density can be supported in a split spectrum network with omnidirectional femtocell antennas. Even though Chandrasekhar suggests time-hopping to reduce outage probability, Chandrasekhar's system is designed specifically for a CDMA-based communication system, which does not use frequency-hopping. Chandrasekhar also does not specify the spreading that is possible, which changes depending on the number of users and their data rates. In orthogonal frequency division multiple access (OFDMA)-based signaling, effective spreading conditions are still not well understood.
Ho analyzed handover probabilities for different power configurations at a femtocell. As manual cell planning used in macrocell networks is not practical for femtocells (i.e., not cost-effective), femtocells typically require auto-configuration capabilities, such as femtocell power and cell size auto-configuration. Using simulations, Ho shows that, in a residential co-channel femtocell deployment, call drop probabilities can be significantly decreased through simple pilot power adaptation mechanisms.
Claussen discloses a simple power control algorithm for pilots and data in a femtocell. Claussen's simulation results show that the interference to the macrocell network can be minimized through such a power control algorithm.
Femtocells have also become popular within standardization groups. For example, the 3GPP standard conducted an extensive study of CDMA-based femtocells. Their results are published in the technical report, entitled “3rd generation partnership project; technical specification group radio access networks; 3G Home NodeB study item technical report,” Shanghai, China, March 2008, 3GPP TR 25.820 V8.0.0 (2008-03), available on-line at: http://www.3gpp.org/ftp/Specs/html-info/25820.htm. The standardization of femtocell-based OFDMA technology became more active in the second half of 2008. One example of such effort is the IEEE 802.16m standard (enhancement to the Mobile WiMAX standard), published as “Support for Femtocell,” by Guang Han, Technical Contribution to IEEE 802.16m, Jul. 7, 2008. Other examples are the Long Term Evolution (LTE)-Advanced standard. See, for example, the studies (a) “Requirements for LTE Home eNodeBs,” published by Orange, Telecom-Italia, T-Mobile, and Vodafone, 3GPP Document R4-070209, Lemesos, Cyprus, March 2007; and (b) “Home eNodeB considerations for LTE,” published by Vodafone-Group, 3GPP Document R4-070456, Sophia Antipolis, France, April 2007. These standard studies include femtocells as possible inclusion in the final versions of their respective standards. Notably, IEEE 802.16m received a larger number of contributions related to handling interference in femtocells. Such contributions include:                (a) “Interference Mitigation by Initial Configuration for Femtocell Access Points in IEEE802.16m Network” (“Y-Zhou I”), by Yuefeng Zhou, Karthik Sundaresan, Honghai Zhang, Nader Zein, and Sampath Rangarajan, Technical Contribution to IEEE 802.16m, Jul. 8, 2008, available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—605.ppt;        (b) “Dynamic Interference Mitigation for Femtocell Access Points in IEEE802.16m Network” (“Y-Thou II”), by Yuefeng Zhou, Karthik Sundaresan, Honghai Zhang, Nader Zein, and Sampath Rangarajan, Technical Contribution to IEEE 802.16m, Jul. 8, 2008, available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—607.ppt;        (c) “Downlink Power Control for WiMAX Femtocell in IEEE 802.16m” (“J-Zhou”), by Jun Zhou, Andreas Maeder, Linghang Fan, Nader Zein, and Tetsu Ikeda, Technical Contribution to IEEE 802.16m, Oct. 30, 2008, available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—1244.pdf;        (d) “Interference Mitigation for Closed User Group Femtocells” (“Saperi”), by Luciano Sarperi and Yanling Lu, Technical Contribution to IEEE 802.16m, Oct. 31, 2008, available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—1315.doc;        (e) “Femtocell Interference Mitigation by Autonomously Adjusting Radio Resource Parameters” (“Morita”), by Motoki Morita, Nader Zein, Jun Zhou, Linghang Fan, and Tetsu Ikeda, Technical Contribution to IEEE 802.16m, Oct. 31, 2008. Available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—1254.pdf;        (f) “Interference Mitigation by Location-based Channel Allocation for Femtocell” (“Chen”), Whai-En Chen, Shiann-Tsong Sheu, Chih-Cheng Yang, Kanchei (Ken) Loa, Yung-Ting Lee, Chiu-Wen Chen, Chun-Yen Hsu, Youn-Tai Lee, Yi-Hsueh Tsai, Tsung-Yu Tsai, Chih-Shin Lin, Yang-Han Lee, and Yih Guang Jan, Technical Contribution to IEEE 802.16m, Oct. 31, 2008, available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—1319.ppt;        (g) “Interference mitigation in Femtocells” (“Zhang”), Kaibin Zhang, Gang Shen, and Jimin Liu, Technical Contribution to IEEE 802.16m, Oct. 31, 2008, available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—1427.ppt; and        (h) “Self-coordinated femtocells to mitigate interference in IEEE 802.16m” (“Kone”), by Mamadou Kone, Ming-Hung Tao, Ying-Chuan Hsiao, and Richard Li, Technical Contribution to IEEE 802.16m, Nov. 6, 2008, available on-line at: http://wirelessman.org/tgm/contrib/C80216m-08—1421r1.ppt        
In Y-Zhou I, a femtocell base station (fBS) obtains initial measurements of the signal strengths and preamble indices of neighbor stations or access points with unique IDs, and indentifies the preamble index with the least received signal strength. This initial information, the highest number of mobile stations (MSs) connected to the fBS and their maximum traffic load are reported to an access server network gateway (ASN-GW) to facilitate a scheduling algorithm in the ASN-GW, which then allocates preamble indices and subchannels to the fBS. In other words, based on measurements reported by the fBSs, the ASN-GW minimizes interference by intelligent scheduling. Similarly, in Y-Thou II, an fBS periodically measure the signal strengths and preamble indices of neighboring base stations (BSs) or access points with unique IDs, and identifies the preamble index with the least received signal strength. The fBS periodically reports the measurements to the ASN-GW to facilitate a scheduling algorithm in the ASN-GW and to facilitate allocation of preamble indices and subchannels to the fBSs.
J-Zhou discloses a downlink close-loop power control scheme for femtocells. In the absence of downlink traffic, each MS connected to a femtocell BS periodically measures and records metrics of interference and received signal intensity for each subcarrier or each subchannel in which signals are expected to be received from the anchored fBS. When a downlink traffic channel is needed, the MS reports the recorded metrics to the fBS via an uplink control channel. Consequently, the fBS allocates power to each user according to QoS, loading, the value of received metric and the interference limitation. To reduce co-channel interference, a WiMAX fBS allocates only the necessary resources in the downlink for every active user.
Saperi discloses a system in which macrocell BSs (mBSs) under the network operator's control impose over the backbone scheduling restrictions (e.g., power control information or fractional frequency reuse (FFR) related information) on closed user group fBSs, so as to minimize interference between the macrocell and the femtocell.
In Morita, an fBS measures interference from surrounding macro or micro cells or neighboring femtocells in order to mitigate interference. Based on the measured surrounding reception power, the fBS selects an appropriate carrier frequency to avoid mutual interference between macro or micro cells and femtocells or among femtocells. Further, the femtocell sets a downlink (DL) maximum transmit power and an UL maximum allowed transmit power for camping MSs in such a way that to maximize the coverage of the femtocell, while keeping constant the interference impact to the surroundings.
Chen discloses using location information to mitigate interference among femtocells. In Chen's system, an operational channel of a femtocell is allocated based on both coarse location information obtained over an Internet connection (or, through another method, such as GPS), and operational channel information of neighboring femtocells.
Zhang discloses that an fBS and a femtocell MS (fMS) measure (initially and periodically) surrounding interference by scanning neighboring femtocells and the macro-cell. The fMS is also able to report its measurements to the fBS, so that interference mitigation techniques may be applied by both femtocell and macrocell users. However, Zhang provides no specific information as to how interference may be canceled.
Kone discloses an mBS that communicates with fBSs over the air and broadcasts a number of profiles selectable by the fBSs. These profiles correspond to the use of different part of DL/UL scenarios. An fBS can scan neighbor fBSs and the mBS to receive the available profiles, frequencies used and other measurement results. Using information derived from the scan, the fBS may decide either to use a different profile at the same frequency or choosing a different frequency with any available profile, so as to mitigate interference with adjacent fBSs.